Prior-art optical displays include back-lighted liquid crystal displays and reflective liquid crystal displays. Back-lighted displays consume more power than reflective displays do. In addition, back-lighted displays are larger and heavier than reflective displays. Thus, back-lighted displays have significant disadvantages for use in portable electronic products.
Reflective displays rely on ambient light to illuminate the pixels, symbols, or characters of the displays. Reflective displays are susceptible to the negative influence of glare that is typically attendant with the ambient light illuminating the display. As used herein, glare or glare light rays refer to an unwanted reflection of incident ambient light off any interface associated with the display device. The glare light rays produce a reflection which has no utility for viewing purposes. Conventional reflectors within displays frequently reflect light such that the direction of maximum brightness of the display is at substantially the same angle as the glare light rays. Consequently, the glare light rays in reflective displays often coincide with a viewer's preferential viewing angle of the display device. The glare greatly reduces the effectiveness of the display by reducing the contrast between inactive and active pixels. Annoyed viewers may need to reorient their viewing angle to view displays away from glare and away from the maximum brightness of the displays.
In a reflective display, ambient light may illuminate a physical viewing interface (i.e. lens) and contribute to the glare perceived by a viewer. The physical viewing interface is preferably coincident with an imaginary viewing plane. The ambient light passes through the viewing plane at one or more incident angles. The incident angles are determined by the propagational direction of the ambient ambient light relative to the viewing plane. The magnitude of the incident angles and the refractive index of the viewing interface determine the extent that ambient light is either transmitted through the viewing plane into the display, or reflected at the viewing plane and potentially perceived as glare by a viewer.
Even the light initially transmitted through the viewing plane may contribute to the glare perceived by a user. The reflective display may have internal refractive interfaces, such as the refractive interfaces associated with the transparent metal electrodes of a twisted nematic display. Such refractive interfaces can reflect some incident light toward the viewing plane and the viewer, reducing the effectiveness of the display.
Holographic reflectors have been placed in reflective liquid crystal displays (LCD's) to ameliorate the affects of glare. While holographic reflectors tend to place the viewing angle in a different direction than the glare, single-layer holographic reflectors tend to act like narrow band filters; hence, appear colored to the user. Multiple-layer holographic filters may be achromatic, but the thickness and additional cost over single layer holographic filters have limited commercial use of the multiple-layer holographic filters.
Therefore, a need exists for a reflective optical display device that minimizes glare degradation, while providing an achromatic, compact and commercially practical display.